Non-impact offset printers require cleaning of the image surface after transferring an image to the paper, and prior to depositing the charge for subsequent images to be printed. Typical transfer processes are not 100% effective, leaving a small amount of toner used to develop the image on the surface of the drum or belt used as the image surface. If this residual toner is not removed, it will contaminate the printer mechanisms and degrade subsequent images.
It also must be understood that cleaning of the dielectric belt in a direct charge deposition imaging system must be done while a very strong electrostatic image remains, because one cannot normally discharge the image surface before cleaning.
In contrast, conventional photoconductive imaging systems use light to reduce the electrical charge holding the residual toner to the imaging surface, and a soft, long nap, rotating brush, or soft plastic scraper to remove the bulk of the residual toner. Ion projection imaging systems that use a drum and pressure transfer use a hard scraper to clean the drum surface. Photoconductive and ion projection imaging systems do not require contact with the image surface other than for cleaning the image transfer. Consequently, these systems are somewhat tolerant of residual toner or paper dust on the image surface after cleaning.
In a direct charge deposition imaging system of the type contemplated by this invention, contact is made between the image surface and the print head to maintain the necessary clearance gap between the print head pins and the surface of the dielectric material of the belt. Any residual material on the belt surface will tend to accumulate at the belt/head interface causing an accumulation of material that can lift the belt off the print head increasing the pin/belt gap to a level that can cause print degradation. For this reason, the cleaning function in a direct charge deposition printing system is more critical than in other non-impact printing systems and is extremely sensitive.
In addition, the charge pattern generating the image in the direct charge deposition process is imposed on a simple dielectric material with no photoelectric properties, so exposure to light to discharge the surface has no effect. In fact, the residual image charge is directly blocked by the tightly bonded residual toner that did not transfer to the paper in the transfer zone, thus making pre-cleaning corona treatment ineffectual.
The use of a scraper to remove toner from the belt of a direct charge deposition printer results in a damage to the dielectric surface of the belt. In use, material can build up on the surface of the scraper to the extent that it lifts the belt away from the scraper and allows a portion of the belt to pass the scraper without being cleaned.
The use of a long-nap rotating brush (normally having bristles about 0.400 inches in length) to clean the belt surface presents two problems. First, since the image charge is still present on the belt, the toner clings to the belt and is simply moved aside as the tip of a soft bristle moves across the surface. Second, if the toner does cling to the bristle, or become trapped within the nap, the depth of the nap is such that it is very difficult to remove the toner from the brush with the normal vacuum and "flicker bar" typically used to clean these brushes. As a result, some of the toner is carried back to the surface of the belt and is redeposited on the residual charge pattern on the belt as the belt exits the cleaning station. In addition, while running a printer at the high speed (about 12 inches per second) contemplated for this invention, frictional heating at the "flicker bar" and long nap brush interface can cause local fusing of toner particles on the "flicker bar" and brush fibers degrading cleaning performance.